1. Field of the Invention
The invention relates to the art of signal convolver devices for generating a convolved output signal from a pair of predetermined input signals. More particularly, the invention pertains to improving the light sensitivity of the signal convolving device by using the photovoltaic effect of a diode array such that the signal convolver can be used as an imaging device.
2. Description of Prior Art
Many devices of prior art exist which perform the convolution of two acoustic wave input signals introduced at opposite ends of an acoustic wave propagation medium having piezoelectric properties. The oppositely propagating acoustic waves W.sub.1, W.sub.2 generated by the application of two signals to the piezoelectric medium are arranged to provide a second-order parametric interaction at an output electrode near the central region of the medium where the waves overlap. The physical realization of convolution C.sub.n of the two time functions f(.tau.), g(.tau.) generated is mathematically represented by EQU C.sub.n =.intg.f(.tau.)g(t-.tau.) d.tau.
where t is representative of the time displacement between the two functions. The applied signals are at such a power level that nonlinear effects result in the wave propagation medium. In particular, a component of electric displacement proportional to the product of the amplitudes of the two modulated acoustic waves in the medium is generated at the point of wave overlap and represents the convolution of the two signals. An output signal will only be observed when the conditions of nonlinear parametric interaction exist, e.g., phase matching and frequency conservation, and thus good signal discrimination is obtained with such a device.
Prior art use of such signal convolvers has concentrated on acoustic convolution only using slablike semiconductor layers either contacting or slightly separated from the wave propagation medium in the interaction region so that the electric displacement field is coupled into the body of the semiconductor material. Furthermore, prior art devices generally use opaque wave propagation mediums permitting only acoustic convolution. However, further modulation of the signal tapped from the semiconductor is possible if the wave propagation medium is made transparent and if a diode array is substituted for the semiconductor slab normally used. A light may then be shined through the transparent wave propagation medium coincident with generation of the acoustic waves W.sub.1, W.sub.2. The photovoltaic effect of the diode array permits modulation of the convolved signal by modulation of light intensity of the light source beamed upon the transparent wave propagation medium, and thus the convolver may be used as an imaging device.